Cytochrome reductase is a large membrane protein complex which comprises the middle section of the mitochondrial respiratory chain. We collected a MAD dataset at 3 wavelengths determined by an X-ray absorption spectrum of the crystals. When phased with DM-improved isomorphous phases, this dataset clearly located the iron centers in the crystal. Phases calculated from the data are inferior to the isomorphous phases, but the data is being used as an independent criterion to monitor goodness of phases in particular resolution shells during phase improvement/extension by the height of peaks in anomalous difference maps phased with the test phases.